1. Field of the Invention
The present invention relates to a collimator to be used in a nuclear medical apparatus such as a SPECT (Single Photon Emission Computed Tomography) apparatus, and a method for manufacturing such a collimator.
2. Description of the Background Art
In a nuclear medical apparatus such as a SPECT apparatus, .gamma. rays emitted from radioactive materials deposited inside a body to be examined are detected, and an image of a distribution of the radioactive materials inside the body is obtained on a basis of the detected .gamma. ray signals, where the obtained image is utilized in the diagnosis of a cancer and a tumor. In such a nuclear medical apparatus, a collimator is attached on a detector device in order to selectively collect the .gamma. rays from the radioactive materials inside the body at the detector device. The .gamma. rays selectively collected at the detector device by using the collimator are then converted into light signals and then into electric signals by using a scintillator, and the obtained electric signals corresponding to the detected .gamma. rays are utilized as the image data in the image reconstruction process.
For such a collimator to be used in a nuclear medical apparatus, there are several types including a parallel hole collimator in which all the holes arranged in an array are parallel to each other, and a single focus (fan beam) collimator in which each hole in an array is provided with a prescribed inclination angle such that the collimator as a whole has a focal line in order to improve the sensitivity and the resolution of the collimator.
In the SPECT apparatus for the head portion diagnosis, three such collimators are used in an arrangement in which each collimator is located on each side of an equilateral triangle formed by detectors arranged around the head portion of a patient.
Among the various types of such a collimator, the parallel hole collimator has conventionally been manufactured by the following methods relatively easily.
(1) A method of folded foil construction in which corrugated thin plates made of lead are piled up to form a collimator body.
(2) A method in which pipe shaped members made of lead are glued together to form a collimator body.
On the other hand, the single focus collimator has been more difficult to manufacture conventionally, because each hole in the array must be manufactured to be oriented toward a single focal line, and the following manufacturing methods have been employed for the single focus collimator conventionally.
(1) A method using pins in which approximately thirty to fifty thousand pins each in a shape of a hole of a collimator to be manufactured are mounted between two templates with pre-manufactured pin positions in an array such that all the pins are oriented toward a predetermined single focal line, and then the lead is casted between the templates with the pins mounted, such that a desired single focus collimator body with all the holes arranged in an array oriented toward the predetermined single focal line can be obtained by pulling out all the pins after the lead casting.
(2) A method using tungsten plates as disclosed in U.S. patent application Ser. No. 07/538,763, in which one type of tungsten plates are provided with fan shape patterned grooves oriented toward a common focal point while the other type of tungsten plates are provided with parallel grooves, such that these two types of tungsten plates can be assembled into a lattice shape by perpendicularly engaging the fan shaped grooves on one type of the tungsten plates with the parallel grooves of the other type of the tungsten plates, so as to form a desired single focus collimator body with all the holes arranged in an array oriented toward the predetermined single focal line.
However, such conventional methods of manufacturing a single focus collimator have been associated with the following problems.
First of all, as for the method using pins, the following three problems exited.
(1) Each of the normally thirty to fifty thousand pins used in manufacturing one single focus collimator must be applied with a tapering process in order to facilitate an easy pulling out operation after the lead casting, so that a number of processes for preparing the pins can be enormously large as well as ineconomical.
(2) Each of the normally thirty to fifty thousand pins used in manufacturing one single focus collimator must be mounted between the templates one by one and them pulled out after the lead casting one by one, all manually, so that the amount of work required for the worker can be enormously large as well as ineconomical.
(3) The precision of the manufactured single focus collimator is often deteriorated by the bending of the very thin templates due to the weights of the pins, and by the inaccuracy of the pin orientation due to the looseness of the fitting of the pins at the pin positions on the templates.
On the other hand, as for the method using tungsten plates, the following two problems exited.
(1) Each plate to form a collimator body is required to have a thickness of approximately 0.2 mm, so that the material for each plate must have a sufficient rigidity to be able to maintain its shape in such a thin thickness, along with a sufficient .gamma. ray shielding property. For this reason, the tungsten is an only presently available metallic material for each plate. However, the tungsten is a rare metal which is very expensive, so that the cost for manufacturing the collimator inevitably becomes very high. In this regard, if the lead which has the sufficient .gamma. ray shielding property and is relatively inexpensive is to be used for the material for each plate, the plate manufactured in a thickness of approximately 0.2 mm would not be able to maintain its shape in the assembling operation because the lead does not have the sufficient rigidity.
(2) In order to cut the tungsten plates to form the grooves thereon, it becomes necessary to utilize the wire cut electric spark manufacturing process because of the high rigidity of the tungsten. However, such a wire cut electric spark manufacturing process is very time consuming, and therefore the cost for manufacturing the collimator inevitably becomes high.
Moreover, the conventional single focus collimator is also associated with the problem that the sensitivity becomes higher in a central region compared with peripheral regions, such that the appropriate correction of the detector output has been necessary.